The present invention relates to improved cathodes for use in electrolytic cells. The cathodes of this invention exhibit low hydrogen overvoltage, improved bonding of the surface layer to the substrate and increased stability under normal cell operating conditions. The cathodes of the present invention are particularly useful in the electrolysis of aqueous solution of alkali metal halides to produce alkali metal hydroxides and halogens, or in the electrolysis of aqueous solutions of alkali metal halides to produce alkali metal halates, or in water electrolysis to produce hydrogen.
In an electrochemical cell, large quantities of electricity are consumed to produce alkali metal hydroxides, halogens, hydrogen, and alkali metal halates in electrochemical processes familiar to those skilled in the art. With increased cost of energy and fuel, the savings of electricity, even in relatively minor amounts, is of great economic advantage to the commercial operator of the cell. Therefore, the ability to effect savings in electricity through cell operation, cell design, or improvement in components, such as anodes and cathodes, is of increasing significance.
In such electrolytic processes, hydrogen is evolved at the cathode, and the overall reaction may be theoretically represented as: EQU 2H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2OH.sup.- ( 1)
However, the cathode reaction actually produces monoatomic hydrogen on the cathode surface, and consecutive stages of reaction (1) can be represented as follows: EQU H.sub.2 O+e.sup.- .fwdarw.H+OH.sup.- EQU 2H.fwdarw.H.sub.2 ( 2)
or as: EQU H.sub.2 O+e.sup.- .fwdarw.H+OH.sup.- EQU H+H.sub.2 O+e.sup.- .fwdarw.H.sub.2 +OH.sup.- ( 3)
The monoatomic hydrogen generated as shown in reactions (2) or (3) is adsorbed on the surface of the cathode and desorbed as hydrogen gas.
The voltage or potential that is required in the operation of an electrolytic cell includes the total of the decomposition voltage of the compound being electrolyzed, the voltage required to overcome the resistance of the electrolyte, and the voltage required to overcome the resistance of the electrical connections within the cell. In addition, a potential, known as "overvoltage" is also required. The cathode overvoltage is the difference between the thermodynamic potential of the hydrogen electrode (at equilibrium) and the potential of an electrode on which hydrogen is evolved due to an impressed electric current. The cathode overvoltage is related to such factors as the mechanism of hydrogen evolution and desorption, the current density, the temperature and composition of the electrolyte, the cathode material and the surface area of the cathode.
In recent years, increasing attention has been directed toward improving the hydrogen overvoltage characteristics of electrolytic cell cathodes. In addition to having a reduced hydrogen overvoltage, a cathode should also be constructed from materials that are inexpensive, easy to fabricate, mechanically strong, and capable of withstanding the environmental conditions of the electrolytic cell. Iron or steel fulfills many of these requirements, and has been the traditional material used commercially for cathode fabrication in the chlor-alkali industry. When a chlor-alkali cell is by-passed, or in an open circuit condition, the iron or steel cathodes become prone to electrolytic attack and their useful life is thereby significantly decreased.
Steel cathodes generally exhibit a cathode overvoltage in the range of from about 300 to about 500 millivolts under typical cell operating conditions, for example, at a temperature of about 100.degree. C. and a current density of between about 100 and about 200 milliamperes per square centimeter. Efforts to decrease the hydrogen overvoltage of such cathodes have generally focused on improving the catalytic effect of the surface material or providing a larger effective surface area. In practice, these efforts have frequently been frustrated by cathodes or cathode coatings which have been found to be either too expensive or which have only a limited useful life in commercial operation.
Various coatings have been suggested to improve the hydrogen overvoltage characteristics of electrolytic cell cathodes in an economically viable manner. A significant number of the prior art coatings have included nickel, or mixtures, alloys or intermetallic compounds of nickel with various other metals. Frequently, when nickel is employed in admixture with another metal or compound, the second metal or compound can be leached or extracted in a solution of, for example, sodium hydroxide, to provide a high surface area coatings, such as Raney nickel coatings.
Representative coatings of the prior art are disclosed in U.S. Pat. No. 3,291,714, issued Dec. 13, 1966, and U.S. Pat. No. 3,350,294, issued Oct. 31, 1967. These patents disclose inter alia cathode coatings comprising alloys of nickel-molybdenum or nickel-molybdenum-tungsten electroplated on iron or steel substrates. The electro-deposition of nickel-molybdenum alloys utilizing a pyrophosphate bath is also discussed by Havey, Krohn, and Hanneken in "The Electrodeposition of Nickel-Molybdenum Alloys", Journal of the Electrochemical Society, Vol. 110, page 362, (1963).
Other attempts have been made in the prior art to produce coatings of this general variety which offer an acceptable compromise between coating life and low overvoltage characteristics. U.S. Pat. No. 4,105,532, issued Aug. 8, 1978, and U.S. Pat. No. 4,152,240, issued May 1, 1979, are representative of these attempts disclosing, respectively, alloys of nickel-molybdenum-vanadium and nickel-molybdenum using specially selected substrate and intermediate coatings of copper and/or dendritic copper. Similar coatings are also disclosed in U.S. Pat. Nos. 4,033,837 and 3,291,714.
The surface treatment of a Raney nickel electrode with a cadmium nitrate solution for the purpose of reducing hydrogen overvoltage has been investigated by Korovin, Kozlowa and Savel'eva in "Effect of the Treatment of Surface Raney Nickel with Cadmium Nitrate on the Cathodic Evolution of Hydrogen", Soviet Electrochemistry, Vol. 14, page 1266 (1978). Although the initial results of such a coating exhibit a good reduction in hydrogen overvoltage, it has been found that the overvoltage increases rapidly to the original level after a short period of operation, i.e. about 2 hours.
U.S. Pat. No. 4,190,516, issued Feb. 26, 1980, discloses an electrolytic cell cathode having low hydrogen overvoltage and high durability. The cathode consists essentially of a base material, i.e. copper, iron or nickel, having a coating of at least one metal of Group VIII of the periodic table. The coating is deposited on the base material from a solution or suspension of a sulfur-containing compound of the Group VIII metal such as nickel thiocyanate. The cathode is then sintered in an electroless process to convert the coating predominately to the Group VIII metal while retaining at least 3% sulfur. Below a sulfur content of 3% the hydrogen overvoltage of the resulting cathode coating is disclosed as being too high to be suitable for use in electrolytic applications.
U.S. Pat. No. 4,251,478, issued Feb. 17, 1981, discloses low overvoltage hydrogen cathodes comprising an electroconductive substrate material, an intermediate imporous nickel layer, and a porous surface comprising a major portion of nickel and a transition metal such as molybdenum. The porous surface layer can also include a leachable material which is codeposited along with the nickel and transition metal. The leachable materials, which include aluminum, cadmium and copper, are removed from the surface layer with sodium hydroxide to increase the surface area, leaving only trace amounts remaining after leaching.
Copending application Ser. No. 104,235, filed Dec. 17, 1979, addresses the problem of low hydrogen overvoltage by disclosing a novel cathode having an active surface layer comprising, as a preferred embodiment thereof, a codeposit of nickel, molybdenum or an oxide thereof, and cadmium. The application also describes various intermediate protective layers, which can be suitably interposed between the substrate and active surface layer to protect the substrate from the corrosive effects of the electrolytic cell environment. Such layers include nickel and various alloys or mixtures of nickel with other metals.
Although many of the cathodes disclosed in the prior art exhibit satisfactory adherence of the coating to the substrate under normal conditions, there exists a continuing need to maximize the useful life of the cathode and to improve the stability of the coating under the conditions prevailing during the commercial operation of an electrolytic cell. Many of the prior art attempts at reducing the hydrogen overvoltage of the cathode, while initially successful, have ultimately failed due to rapid deterioration of the coating in the caustic environment, causing the coating to separate from the substrate material.